This invention relates generally to an optical element, and more particularly to optical elements consisting of an isometric crystal such as calcium fluoride (CaF2), barium fluoride (BaF2), and strontium fluoride (SrF2), which elements are used in the short wavelength range from vacuum ultraviolet radiation to deep ultraviolet radiation, and a manufacturing method therefor.
The demand for finer semiconductor devices mounted in electronic apparatuses increases more and more as the need for smaller and thinner electronic apparatuses grows in recent years, and in order to fulfill the demand, various proposals for higher exposure resolution have been made. Since shortened wavelength of an exposure light source would serve as an effective means for achieving higher resolution, an ArF excimer laser (approximately 193 nm in wavelength) has been replacing a KrF excimer laser (approximately 248 nm in wavelength) for the exposure light source in recent years, and an F2 excimer laser (approximately 157 nm in wavelength) has become commercially practical. The calcium fluoride (CaF2) single crystal is most suitably used as an optical material for optical elements such as lenses and diffraction gratings for use with an exposure optical system in that light transmittance (i.e., internal transmittance) in the above wavelength range is high. In addition, the barium fluoride (BaF2) single crystal and strontium fluoride (SrF2) single crystal are also high in transmittance, thus their application has been under consideration.
The calcium fluoride (CaF2) single crystal has been manufactured up to this time by the crucible descent method (also known as “Bridgman method”). In this method, materials for crystalline substances are filled in a crucible, melted by application of heat using a heater, and cooled by lowering the materials in the crucible, whereby the materials are crystallized. In this process a seed crystal having a desired face orientation to induce growth of a crystal is placed at a bottom of the crucible, so that the face orientation of the crystal to be grown is controlled. The calcium fluoride is generally advantageous in easy handling of cleavage plane of a <1 1 1> face perpendicular to the crystal orientation [1 1 1] axis, and further in less effect of birefringence due to photoelasticity with respect to light beams incident from a direction of the [1 1 1] axis. During the manufacturing process, the calcium fluoride grows to form a desired optical element under control of the face orientation using the seed crystal oriented to the <1 1 1> face, or by slicing the <1 1 1> face of the crystal, and thereby the optical element having a light beam incident surface at the <1 1 1> face is obtained. Consequently, an optical system constituted of the above optical element has an optical axis identical with the [1 1 1] axis, to contribute to improved optical properties of the optical element (or optical system).
However, even if an ideal calcium fluoride crystal without stress strain could grow, the use of a light beam with shorter wavelength would make potential birefringence (intrinsic birefringence) non-negligible, and the intrinsic birefringence would disadvantageously deteriorate the optical properties of the optical element and thus of the image-forming quality of the exposure apparatus. These disadvantages came into focus by Burnett, et al.'s report in the 2nd International Symposium on 157 nm Lithography held in Dana Point, Calif. in May 2001. Referring to their experimental results, the maximum value of the intrinsic birefringence of the calcium fluoride is 6.5 nm/cm with the wavelength of 157 nm, and the directions in which the maximum value is exhibited are twelve directions including an [1 1 0] axis direction. In the conventional exposure apparatuses, or projection optical systems in particular, the above-described optical element using the <1 1 1> face has an optical axis identical with the [1 1 1] axis, which is only 35.26 degrees apart from the [1 1 0] axis exhibiting the maximum birefringence. For example, the numerical aperture (NA) of the calcium fluoride for a beam passing along the [1 1 0] axis is 0.87 with the wavelength of 193 nm, and 0.90 with the wavelength 157 nm. Particularly, exposure apparatuses in recent years have been required to achieve improvement in resolution and thus tend to include a projection optical system with increased NA, and the NA ranging between 0.65 and 0.80 has become the mainstream. Accordingly, in the optical element of the projection optical system near wafer where the maximum angle of a light beam becomes the greatest, the light beam passes through an area near the [1 1 0] axis where the intrinsic birefringence represents a large value. This makes it difficult for the conventional exposure apparatuses to perform exposure of good image-forming quality.